Haskell: How to make a type an instance of two typeclasses?

Question

Is it possible to make Level an instance of Eq and Ord?

instance Eq Ord Level where
    compare First Second = LT
    ...

I've tried this

instance (Ord, Eq) => Level where ...

But I've got an error

‘compare’ is not a (visible) method of class ‘Level’

I know I can use deriving (Eq) on my data Level. But I cannot change my data.

chi · Accepted Answer · 2016-03-24 22:41:01Z

7

Use two instances

instance Eq Level where
   x == y = ...
instance Ord Level where
   compare x y = ...

Alternatively, use the standalone deriving extension:

{-# LANGUAGE StandaloneDeriving #-}
...       -- ^^^^^^^^^^^^^^^^^^ must be at the top of the file
deriving instance (Eq Level)
deriving instance (Ord Level)

answered Mar 24, 2016 at 22:41

chi

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1 Comment

dfeuer Over a year ago

@chi, ah, missed it. Sorry.

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